[InstCombine] Simplify a umul overflow check to a != 0 && b != 0.

This patch adds a simplification if an OR weakens the overflow condition
for umul.with.overflow by treating any non-zero result as overflow. In that
case, we overflow if both umul.with.overflow operands are != 0, as in that
case the result can only be 0, iff the multiplication overflows.

Code like this is generated by code using __builtin_mul_overflow with
negative integer constants, e.g.
   bool test(unsigned long long v, unsigned long long *res) {
     return __builtin_mul_overflow(v, -4775807LL, res);
   }

```
----------------------------------------
Name: D74141
  %res = umul_overflow {i8, i1} %a, %b
  %mul = extractvalue {i8, i1} %res, 0
  %overflow = extractvalue {i8, i1} %res, 1
  %cmp = icmp ne %mul, 0
  %ret = or i1 %overflow, %cmp
  ret i1 %ret
=>
  %t0 = icmp ne i8 %a, 0
  %t1 = icmp ne i8 %b, 0
  %ret = and i1 %t0, %t1
  ret i1 %ret
  %res = umul_overflow {i8, i1} %a, %b
  %mul = extractvalue {i8, i1} %res, 0
  %cmp = icmp ne %mul, 0
  %overflow = extractvalue {i8, i1} %res, 1

Done: 1
Optimization is correct!

```

Reviewers: nikic, lebedev.ri, spatel, Bigcheese, dexonsmith, aemerson

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D74141